1. Field of the Invention
The present invention relates in general to soldering or brazing for joining a ceramic member metallic member together and more particularly to a ceramic base and metallic member assembly for electronic devices, including a ceramic base substrate and a metallic member such as an I/O (input/output) terminal, a seal ring, etc. joined to the ceramic base substrate with solder.
2. Description of the Prior Art
Alumina substrates (thermal expansion coefficient .alpha.=6.8.times.10.sup.-6 /.degree.C.) have heretofore been used for most IC (integrated circuit) packages for accommodating therewithin and hermetically sealing a semiconductor chip or chips. Metallic members such as I/O terminals and a seal ring are joined to the ceramic substrate by first forming a thick film or thin film metallized layer on the ceramic substrate and then joining the metallic members made of kovar (trademark, 29%Ni--17%Co--Fe alloy; .alpha.=4.7.times.10.sup.-6 /.degree.C.) or 42 alloy (42%Ni--Fe alloy; the metallized layer with silver solder (eutectic Ag solder, etc.).
However, in view of the recent trend of rising signal frequencies and the development of larger power chips, the alumina substrate is encountered by a problem that since its dielectric constant is relatively large the electrostatic capacitance in the signal line becomes so large as to cause delay in signal propagation or the chip is subjected to a thermal stress due to the difference in thermal expansion coefficient between the alumina substrate and the silicon semiconductor chip (.alpha.=3.0.about.3.5.times.10.sup.-6 /.degree.C.).
In order to meet with the requirement for a high density and high speed in operation of semiconductor devices in an IC package, glass ceramic (.alpha.=1.5.about.5.0.times.10.sup.-6 /.degree.C.), aluminium nitride (.alpha.=4.4.times.10.sup.-6 /.degree.C.), mullite (.alpha.=3.8.times.10.sup.-6 /.degree.C.), etc., which feature to be low in dielectric constant and approximate in thermal expansion coefficient to a silicon semiconductor chip, have been examined with a view to substituting for alumina.
While these materials approximate in thermal expansion coefficient to the silicon semiconductor chip, they are lower in strength as compared with alumina ceramic, so that a crack or cracks are liable to occur at or adjacent the joining portion due to the difference in thermal expansion coefficient between the ceramic substrate and the silicon semiconductor chip.
Further, the thermal stress caused in the ceramic base by soldering or brazing is a compression stress when the metallic member is smaller in thermal expansion coefficient than the ceramic substrate and reversely a tensile stress when the metallic member is larger in thermal expansion coefficient than the ceramic substrate.
In general, ceramic is strong against a compression stress but weak against a tensile stress. This will turn out a disadvantage in case ceramic, which is low in thermal expansion coefficient, is joined to a metallic member.
As described above, it has been impossible to attain good joining of the ceramic substrate and the metallic member when glass ceramic, etc. is used for the ceramic substrate.
In this connection, JP-B-4-64466 (publication of Japanese patent application after examination) discloses to solve the above problem by forming a stress relief layer made up of copper, iron, etc. and having the thickness of 1.about.20 .mu.m, preferably about 6 .mu.m on the joining layer affixed to the ceramic substrate.
Further, JP-A-64-1268 (publication of Japanese patent application before examination) discloses that it is possible to join I/O terminals to a metallized layer on a ceramic substrate at a relatively low temperature with solder containing copper.
However, with such a stress relief layer as disclosed in JP-B-4-64466, a desired standard defining the joining strength between a ceramic substrate and a metallic member such as an I/O terminal could not be satisfied. Further, the invention disclosed by JP-B-4-64466 is directed to Au--Sn soldering which is a low temperature soldering.
Generally, after the ceramic substrate has joined thereto I/O terminals, etc., soldering is further performed to attach a semiconductor device to the ceramic substrate. Still further, soldering is performed to attach a lid to the ceramic substrate for thereby hermetically sealing the package.
Accordingly, the higher the temperature at which I/O terminals, etc. are joined by solder or brazing metal to a ceramic substrate becomes, the larger the freedom in the subsequent soldering processes becomes. This is desirable since the handling of the substrate can be easier. However, by brazing with silver solder (about 800.about.900 .degree. C.) which is usually used as high temperature soldering or brazing, the standard of the joining strength can not be satisfied.
JP-A-64-1268 discloses a ceramic base and metallic member assembly in which the thermal expansion coefficient of the ceramic is relatively large (supposed to be .alpha.=5.5 .times.10.sup.-6 /.degree.C. or so). However, in case the ceramic for the base was smaller in thermal expansion coefficient (.alpha..ltoreq.5.0 .times.10.sup.-6 /.degree.C.), the joining strength defined in the above described standard could not be attained by only joining with solder containing copper, which is represented by silver solder (e.g., 72Ag-28Cu solder).